# Mechanical Behavior of Steel Fiber-Reinforced Lightweight Concrete Exposed to High Temperatures

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Study

#### 2.1. Raw Materials

#### 2.2. Mix Proportioning and Preparation of Test Specimens

#### 2.3. Heat Treatment

#### 2.4. Physical and Mechanical Test

## 3. Test Results and Discussions

#### 3.1. Visual and Ultrasonic Inspection of Heated Specimens

#### 3.2. Splitting Tensile and Axial Compressive Strength

#### 3.3. Failure Modes after Axial Compression and Compressive Stress–Strain Curves

#### 3.4. Elastic Modulus and Compressive Peak Strains

#### 3.5. Energy Absorption Capacity (Toughness)

_{c0}of the descending branch, known as “0.33f

_{c0}point”), and the concrete energy absorption capacity (toughness) under uniaxial compression was determined based on the area under stress–strain curve before the 0.33f

_{c0}point. Figure 12 shows the toughness test results under different temperatures. The coefficient of variation of the tests was generally less than 15%. and for some cases the coefficient was slightly higher than 15%. However, for all cases the coefficient of variation was less than 20.6%. Figure 12 demonstrating the energy absorption capacity of NWC was higher than LWC at room temperature. After adding steel fiber, the energy absorption capacity of the LWC is enhanced, and the toughness improvement of LWC by adding HF was the most significant. With the temperature increase, the energy absorption capacity of all the concrete increased first and then decreased. All the concrete samples reached the correspondingly most massive energy absorption capacity at a temperature of 400 °C, except for HFAL. By comparing the data given in Figure 12, it can also be found that, after exposure temperature at 800 °C, the energy absorption capacity of the SLWC and NWC, respectively, decreased to 45.5% and 67.7% compared to that at room temperature. In contrast, the energy absorption capacity of ALWC was 40% higher than that at room temperature. The corresponding percentages for the SLWC reinforced by CF and HF were 71.4% and 83.3%, respectively, while those for the ALWC reinforced by CF and HF were 135.7% and 126.3%, respectively. The above phenomenon shows that the concrete residual energy absorption capacity under high temperatures was primarily related to the aggregate type.

## 4. Numerical Models

#### 4.1. High-Temperature Property Relationships

_{c}), splitting tensile strength reduction factor (β

_{t}), elastic modulus reduction factor (β

_{e}), and peak compressive strain amplification factor (β

_{p}) of the NWC, LWC, and SFLWC are derived and given in Table 6. It is noted that when similar β values are inferred from different concrete types, the unified expressions are given.

#### 4.2. Equation of the Compressive Stress–Strain Curve

_{p,T}, and ε and σ are the compressive strain and stress of concrete, respectively; f

_{c,T}and ε

_{p,T}are the peak stress and peak strain of concrete considering the effect of temperature, respectively; n = E

_{c,T}ε

_{p,T}/(E

_{c,T}ε

_{p,T}− f

_{c,T}) and α = E

_{c,T}/E

_{p,T}; n (or α) and φ (or δ) are the shape parameters controlling the rising section and the descending section. E

_{c,T}and E

_{p,T}are initial elastic and peak secant modulus, respectively. However, α ≥ 1 can be inferred from E

_{p,T}≥ E

_{c,T}>0. Through the regression analysis of the measured curve, Table 7 shows the values of the equation parameters.

## 5. Conclusions

- The evolution of the residual compressive properties of concretes after thermal treatment is mainly related to the aggregate type. It should also be noted that, in the present study, the amount of fly ash and silica fume in the concrete mixes is very small. However, further research is required to clarify the effects of mineral additives and amount of aggregate on the post-fire behavior of concrete.
- The presence of steel fibers improved the tensile strength of both pre- and post-fire exposure. SFLWC with hooked end steel fiber lost a lesser amount of strength, which is attributed to the stronger reinforcing action to bridge the cracks. The steel fiber-reinforced ALWC had higher residual compressive and tensile strengths after heating and increased ductile performance due to the homogenous characteristics of the concrete matrix.
- Steel fibres improve the compression absorbed energy of LWC at room temperature as well as at high temperatures. The energy absorption capacity of most concrete mixes increases for temperatures below 400 °C, but decreases above 400 °C. However, the influence of fibers on the residual elasticity modulus is minimal.
- Simple models have been proposed to characterize the material properties as a function of temperature. The regression coefficient were all above 0.96, which indicates that the models were in good agreement with the experimental results. A numerical model is also established to predict the compressive stress–strain relationships of the heated and unheated SFLWC. The theoretical fitting curve complies well with the test results.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

ALWC | All-lightweight concrete |

CF | Crimped shape steel fiber |

CFAL | All-lightweight concrete with crimped shape steel fiber |

CFSL | Semi-lightweight concrete with crimped shape steel fiber |

HF | Hooked end steel fiber |

HFAL | All-lightweight concrete with Hooked end steel fiber |

HFSL | Semi-lightweight concrete with Hooked end steel fiber |

HSC | High strength concrete |

LVDT | Linear Variable Differential Transformer |

LWA | Lightweight aggregate |

LWC | Lightweight concrete |

MLWC | Multi-walled carbon nanotubes reinforced LWC |

NWC | Normal-weight concrete |

RPC | Reactive powder concrete |

SCC | Self-consolidating concrete |

SCC-S | Steel fiber-reinforced self-consolidating concrete |

SFLWC | Steel fiber-reinforced lightweight concrete |

SLWC | Semi-lightweight concrete |

UPV | Ultrasonic pulse velocity |

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**Figure 1.**Lightweight aggregates (LWAs) and steel fibers used in the experiment: (

**a**) shale ceramsite sand; (

**b**) crushed shale ceramsite; (

**c**) sintered fly ash ceramsite; (

**d**) crimped shape steel fiber; (

**e**) hooked end steel fiber.

**Figure 3.**Physical and mechanical tests: (

**a**) Ultrasonic pulse velocity (UPV) test; (

**b**) splitting tension test; (

**c**) axial compression test.

**Figure 4.**Visual inspection of specimens after exposure to high temperatures: (

**a**) the color change of concrete surface after exposure to high temperatures; (

**b**) surface cracks of the non-fiber concrete specimen after exposure to 800 °C; (

**c**) surface cracks of the added-fiber concrete specimen after exposure to 800 °C.

**Figure 5.**Ultrasonic pulse velocities of the studied concrete after the fire: (

**a**) absolute UPV; (

**b**) relative UPV.

**Figure 6.**Temperature effect on compressive strength: (

**a**) absolute compressive strength; (

**b**) relative compressive strength.

**Figure 7.**Temperature effect on splitting tensile strength: (

**a**) absolute splitting tensile strength; (

**b**) relative splitting tensile strength.

**Figure 8.**Failure mode of mixes after compression: (

**a**) All-lightweight concrete (ALWC); (

**b**) Semi-lightweight concrete (SLWC); (

**c**) All-lightweight concrete with crimped shape steel fiber (CFAL); (

**d**) All-lightweight concrete with Hooked end steel fiber (HFAL); (

**e**) Semi-lightweight concrete with crimped shape steel fiber (CFSL); (

**f**) Semi-lightweight concrete with hooked end steel fiber (HFSL).

**Figure 9.**Stress–axial strain curves of all mixes exposed to elevated temperatures: (

**a**) 25 °C; (

**b**) 200 °C; (

**c**) 400 °C; (

**d**) 600 °C; (

**e**) 800 °C.

**Figure 10.**Temperature effect on the elastic modulus: (

**a**) absolute elastic modulus; (

**b**) relative elastic modulus.

**Figure 11.**Temperature effect on compressive peak strains: (

**a**) absolute compressive peak strains; (

**b**) relative compressive peak strains.

**Figure 14.**Evolution of the residual mechanical characteristics under high temperatures: (

**a**) compressive strength; (

**b**) splitting tensile strength; (

**c**) elastic modulus; (

**d**) compressive peak strains.

**Figure 15.**Comparison of the developed stress–strain relationships and experimental results: (

**a**) NWC; (

**b**) ALWC; (

**c**) CFAL; (

**d**) HFAL; (

**e**) SLWC; (

**f**) CFSL; (

**g**) HFSL.

Sample | Chemical Composition (%) | Blaine Fineness (cm ^{2}/g) | Density (g/cm ^{3}) | LOI (%) | ||||
---|---|---|---|---|---|---|---|---|

SiO_{2} | Al_{2}O_{3} | CaO | Fe_{2}O_{3} | MgO | ||||

Cement | 21.66 | 5.42 | 63.15 | 2.62 | 2.89 | 3110 | 3.14 | 1.63 |

Fly ash | 49.10 | 36.70 | 4.96 | 3.67 | 0.37 | 3871 | 2.21 | 2.08 |

Silica fume | 95.28 | 0.28 | 0.35 | 0.14 | 0.13 | 200,000 | 2.20 | 1.40 |

Aggregate | Fineness Modulus | Particle Size/(mm) | Apparent Density/(Kg/m^{3}) | Water Absorption in 24 h/(%) |
---|---|---|---|---|

Crushed shale ceramsite | - | 5–20 | 1390 | 7.4 |

Shale ceramsite sand | 3.2 | ≤5 | 1460 | 20.8 |

Sintered fly ash ceramsite | - | 4–16 | 1420 | 11.3 |

River sand | 2.7 | ≤5 | 2670 | 1.2 |

Shape | Length/(mm) | Equivalent Diameter/(mm) | Aspect Ratio | Tensile Strength/(MPa) | Density/(kg/m^{3}) |
---|---|---|---|---|---|

Hooked end | 35 | 0.50 | 70 | 1200 | 7800 |

Crimped | 30 | 0.55 | 56 | 800 | 7800 |

Concrete Type | Water | Cement | Silica Fume | Fly Ash | Coarse Aggregate | Fine Aggregate | Super Plasticizer | Steel Fiber |
---|---|---|---|---|---|---|---|---|

NWC | 187 | 390 | - | - | 1154 | 663 | - | - |

ALWC | 154 | 450 | 20 | 80 | 489 | 476 | 8.0 | - |

CF-AL | 154 | 450 | 20 | 80 | 489 | 476 | 8.4 | 78 |

HF-AL | 154 | 450 | 20 | 80 | 489 | 476 | 8.7 | 78 |

SLWC | 167 | 390 | 14 | 60 | 439 | 821 | 5.4 | - |

CF-SL | 167 | 390 | 14 | 60 | 439 | 821 | 5.8 | 78 |

HF-SL | 167 | 390 | 14 | 60 | 439 | 821 | 6.1 | 78 |

Concrete Type | Oven Dried Density/(kg/m^{3}) | ${\mathit{f}}_{\mathit{c}}/\mathbf{MPa}$ | ${\mathit{f}}_{\mathit{s}\mathit{p}}/\mathbf{MPa}$ | UPV/(m/s) | E_{c}/GPa |
---|---|---|---|---|---|

NWC | 2361 | 39.36 | 3.72 | 4496 | 28.19 |

ALWC | 1752 | 48.73 | 3.33 | 4016 | 18.28 |

CF-AL | 1859 | 49.71 | 4.17 | 4090 | 17.51 |

HF-AL | 1862 | 51.38 | 4.73 | 4098 | 18.68 |

SLWC | 1888 | 43.24 | 3.83 | 4253 | 20.25 |

CF-SL | 2007 | 45.87 | 4.11 | 4268 | 20.27 |

HF-SL | 1954 | 48.58 | 4.96 | 4270 | 20.43 |

Property | Relation | Concrete Type |
---|---|---|

Compressive strength | ${\beta}_{c}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ -0.959{\left(\frac{T}{1000}\right)}^{2}-0.293\left(\frac{T}{1000}\right)+1.020200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | NWC SL CFSL HFSL (R ^{2} = 0.96) |

${\beta}_{c}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ -1.248{\left(\frac{T}{1000}\right)}^{2}+0.219\left(\frac{T}{1000}\right)+0.997200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | AL CFAL HFAL (R ^{2} = 0.98) | |

Splitting tensile strength | ${\beta}_{t}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ -0.113{\left(\frac{T}{1000}\right)}^{2}-1.007\left(\frac{T}{1000}\right)+1.042200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | NWC AL SL (R ^{2} = 0.99) |

${\beta}_{t}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ -1.008{\left(\frac{T}{1000}\right)}^{2}-0.156\left(\frac{T}{1000}\right)+1.018200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | CFAL CFSL (R ^{2} = 0.96) | |

${\beta}_{t}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ -1.318{\left(\frac{T}{1000}\right)}^{2}+0.252\left(\frac{T}{1000}\right)+0.980200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | HFAL HFSL (R ^{2} = 0.99) | |

Elastic modulus | ${\beta}_{e}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ 1.075{\left(\frac{T}{1000}\right)}^{2}-2.177\left(\frac{T}{1000}\right)+1.069200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | NWC (R ^{2} = 0.99) |

${\beta}_{e}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ 0.021{\left(\frac{T}{1000}\right)}^{2}-1.179\left(\frac{T}{1000}\right)+1.055200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | AL CFAL HFAL (R ^{2} = 0.98) | |

${\beta}_{e}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ 0.353{\left(\frac{T}{1000}\right)}^{2}-1.566\left(\frac{T}{1000}\right)+1.071200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | SL CFSL HFSL (R ^{2} = 0.98) | |

Compressive peak strains | ${\beta}_{p}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ 4.694{\left(\frac{T}{1000}\right)}^{2}+0.332\left(\frac{T}{1000}\right)+0.946200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | NWC (R ^{2} = 0.99) |

${\beta}_{p}=\left\{\begin{array}{c}1.025\xb0\mathrm{C}\\ 2.873{\left(\frac{T}{1000}\right)}^{2}+0.491\left(\frac{T}{1000}\right)+0.971200\xb0\mathrm{C}\le T\le 800\xb0\mathrm{C}\end{array}\right.$ | AL SL CFAL CFSL HFAL HFSL (R ^{2} = 0.98) |

Mix | Parameters | 25 °C | 200 °C | 400 °C | 600 °C | 800 °C |
---|---|---|---|---|---|---|

NWC | n | 3.702 | 8.378 | 9.106 | 7.392 | 5.457 |

φ | 2.342 | 2.945 | 5.328 | 4.393 | 3.867 | |

AL | α | 1.000 | 1.045 | 1.000 | 1.205 | 1.243 |

δ | 5.155 | 7.701 | 3.820 | - | - | |

φ | - | - | - | 4.981 | 10.040 | |

SL | α | 1.000 | 1.000 | 1.000 | 1.156 | 1.104 |

δ | 4.299 | 4.794 | 1.846 | - | - | |

φ | - | - | - | 3.282 | 5.289 | |

CFAL | α | 1.065 | 1.186 | 1.026 | 1.270 | 1.364 |

φ | 8.657 | 8.548 | 3.423 | 4.731 | 6.809 | |

CFSL | α | 1.262 | 1.247 | 1.219 | 1.363 | 1.286 |

φ | 5.402 | 3.513 | 2.107 | 3.398 | 4.451 | |

HFAL | α | 1.112 | 1.130 | 1.041 | 1.449 | 1.333 |

φ | 4.127 | 2.067 | 3.504 | 4.762 | 5.877 | |

HFSL | α | 1.237 | 1.288 | 1.300 | 1.375 | 1.208 |

φ | 2.853 | 3.014 | 2.508 | 3.323 | 3.514 |

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**MDPI and ACS Style**

Wang, H.; Wei, M.; Wu, Y.; Huang, J.; Chen, H.; Cheng, B.
Mechanical Behavior of Steel Fiber-Reinforced Lightweight Concrete Exposed to High Temperatures. *Appl. Sci.* **2021**, *11*, 116.
https://doi.org/10.3390/app11010116

**AMA Style**

Wang H, Wei M, Wu Y, Huang J, Chen H, Cheng B.
Mechanical Behavior of Steel Fiber-Reinforced Lightweight Concrete Exposed to High Temperatures. *Applied Sciences*. 2021; 11(1):116.
https://doi.org/10.3390/app11010116

**Chicago/Turabian Style**

Wang, Huailiang, Min Wei, Yuhui Wu, Jianling Huang, Huihua Chen, and Baoquan Cheng.
2021. "Mechanical Behavior of Steel Fiber-Reinforced Lightweight Concrete Exposed to High Temperatures" *Applied Sciences* 11, no. 1: 116.
https://doi.org/10.3390/app11010116